The present invention relates to methods of producing nonwoven fabrics generally, and more specifically to improved methods of producing apertured webs having a patterned design by means of a hydroentanglement process.
A variety of methods for producing apertured webs are known in the art. According to some methods air or liquid streams are employed to deposit fibers on a web surrounding solid protuberances which are used create apertures in the fibrous web. Kalwaites, U.S. Pat. No. 2,862,251 relates to hydroentanglement methods for production of nonwoven products wherein the application of fluid forces rearranges a layer of fibrous material, such as a web of fibers into a foraminous unitary nonwoven fabric structure comprising spaced, interconnected packed fibrous portions of the starting material, and openings arranged in a predetermined pattern which are separated by the interconnected packed portions. Specifically, a layer of fibrous material such rayon or cotton fibers is positioned between rigid means defining spaced apertures arranged in a pattern such as an apertured plate and tensioned flexible means defining foramina smaller than the apertures such as a fine woven screen. According to one embodiment, the impingement of fluid projected from fluid jets through the apertured plate onto the fibrous layer displaces the fibers laterally away from the apertures to form an apertured nonwoven fabric having apertures corresponding with the apertures of the apertured plate.
Griswold, U.S. Pat. No. 3,025,585 discloses hydroentanglement processes wherein a layer of irregularly arranged fibers is placed upon the free ends of a group of tapered projections arranged in a predetermined pattern upon a permeable backing member with interconnected fiber accumulating spaces between them. Streams of water are then directed against the layer and the fibers are deflected to produce a nonwoven fabric having apertures corresponding to the tapered projections. According to some embodiments of this invention, the tapered projections are attached to a permeable screen. According to other embodiments a single wire of a woven wire screen forms tapered projections as it passes over and under successive cross wires. Variations upon these embodiments utilizing woven screens are widely used in hydroentanglement procedures for use in production of nonwovens.
Evans, U.S. Pat. No. 3,485,706 discloses a nonwoven fabric having a pattern of apertures produced by a hydroentanglement process wherein fibers are deposited on an apertured patterning member such as a fine-wire screen or perforated plate and liquid is jetted at high pressure onto the fibrous layer to entangle the fibers in a pattern determined by the supporting member. The patent further discloses use of patterning members having apertures of random location, size and/or shape for production of non-woven fabrics which do not have regular patterns. Such patterning members are prepared by bonding grains of sand of varying sizes and shapes together so as to leave apertures between the grains. The patent further discloses treating a screen with resin to provide an arrangement of raised lines, filled holes or partially-filled holes, which may be non-repeating for a considerable distance or completely random.
Disclosures of other types of hydroentanglement processes include those of Gilmore et al., European Patent Application Publication No. 418,493 which relates to a nonwoven fabric which is produced by directing high velocity jet streams of water onto a web of fibers using a perforated drum as an aperturing member. The drum can be a cylinder having predetermined diameter and length with a repeating pattern of projections and a plurality of perforations for drainage. The projections are configured such that apertures may be formed in the web of fibers with high efficiency and the nonwoven fabric may be readily peeled off.
Phillips et al., U.S. Pat. No. 5,204,158 disclose an irregularly patterned nonwoven fabric. According to the method of producing the fabric, a fibrous web is caused to be displaced out of registry with the forming member between fluid impacts by hydroentanglement jets.
Despite the variety of hydroentanglement processes known to the art the processes are typically limited in one manner or another such as by cost, poor bonding, lack of aperture clarity, process reliability (e.g., reliable removal of web from the belt without damaging the web) and the like. Methods for production of hydroentanglement fiber webs involving metal rollers with projections as impingement substrates are limited in that the projections must be tapered thus limiting the size/spacing combinations possible. Moreover, certain complex apertured nonwoven designs may be impractical given current machining capabilities. Hydroentanglement processes making use of conventional woven screens are limited by both the patterns and surface topography of the woven filaments. Because the raised "knuckles" on woven screens are not sharply defined the definition of the resulting apertures is similarly and further degraded. In addition, the utility of conventional filament and filament-type screens is limited with respect to the patterns which can be generated. Specifically, when using woven filament screens, aperture size, distance between apertures and total open area of the apertures are dependent variables. This is because thicker filaments or wires result in increased aperture size, but also result in increased distance between individual apertures and a net decrease in aperture area in the resulting nonwoven web.
Accordingly, there remains a desire in the art for efficient methods of producing apertured nonwoven materials characterized by improved flexibility in aperture patterning including increased aperture size and area. Apertured webs characterized by the combination of large, closely-spaced, well-defined, uniformly sized (as a result of being formed on projections having solid elevated portions characterized by a periphery steeply sloped relative to the surface of the foraminous element and further characterized by a distinct upper edge as distinguished from being formed on a highly tapered projection such as formed by the "knuckles" of woven screens) apertures would prove useful as topsheets in absorbent articles in providing for rapid fluid transfer of materials such as runny bowel movements. Runny bowel movement leakage in baby diapers represents a specific problem in the baby diaper art. The problem is particularly significant in the smaller sizes. Accordingly, there exists a need in the art for improved methods of producing apertured webs by means of hydroentanglement processes.
Of interest to the present invention are the disclosures of Johnson et al., U.S. Pat. No. 4,514,345, Smurkoski et al., U.S. Pat. No. 5,098,522 and Trokhan, U.S. Pat. Nos. 4,528,239 and 5,245,025 which disclose methods for making foraminous members, the foramina of which form a preselected pattern. The Johnson patent generally discloses taking a foraminous element such as a screen and using photosensitive resins to construct about and in the foraminous element a solid, polymeric framework which delineates the preselected pattern of gross foramina. Specifically, the method comprises supplying three solid, usually planar, usually continuous materials; a foraminous element such as a woven screen; a backing film such as a thermoplastic sheet; and a mask provided with transparent and opaque regions, the opaque regions of which define the desired, preselected pattern of gross foramina. A fourth material is a liquid photosensitive resin which cures under the influence of light of a particular activating wavelength to form a relatively insoluble, relatively durable, polymeric solid. A coating of the liquid photosensitive resin is applied to the foraminous element, the mask is juxtaposed in contacting relation with the surface of the liquid photosensitive resin and the resin is exposed through the mask to light of an activating wavelength. Curing, as evidenced by solidification of the resin, is induced in those regions of the coating which are exposed to the activating light. Following exposure to light, the backing film and the mask are stripped away from the composite comprising the foraminous element and the resin. Finally, the uncured, still liquid photosensitive resin is removed from the composite by washing leaving behind the desired foraminous member the gross foramina of which define the desired preselected pattern. The Johnson patent discloses that the foraminous member produced by the process of the invention may be used in the production of an improved paper web utilizing a Fourdinier Wire paper making apparatus such that the paper making fibers in the embryonic paper web are deflected into the gross foramina of the foraminous member and the resulting paper web is a continuous web characterized by a plurality of protuberances. Of interest is the disclosure in FIG. 4 of the Johnson patent of a "negative" foraminous pattern defined by discontinuous cured resin forms. The short cellulose fibers used in papermaking react very differently than long synthetic fibers typically used in hydroentangling to produce nonwoven fabrics. Synthetic fibers such as those used in nonwoven fabrics tend to spring upwardly or away from the surface of foraminous elements following hydroentangling. Short cellulose fibers in paper production, such as those used in the Johnson patent, instead exhibit a wet collapse which means that the cellulose fibers generally do not spring upwardly as much as synthetic fibers after being formed into a web. Because synthetic fibers generally do not exhibit such a wet collapse, synthetic fibers typically do not lie as flat between projections after hydroentangling as papermaking fibers lay after settling from the slurry.
Also, whereas fibers for hydroentangling are hit with streams of fluid to form a nonwoven fabric, the cellulose fibers used for papermaking are suspended in a slurry that settles to form a web. The cellulose fibers are not hit with streams of water during paper web formation. Further, nonwoven fabrics are produced from fiber batts or mats that are laid upon the foraminous element prior to hydroentangling. In contrast, the fibers used in papermaking are in a slurry prior to contacting a foraminous element. Trokhan, U.S. Pat. No. 4,528,239, for example, discloses deposition of a fiber slurry onto a foraminous element.